Crude oil generally leaves well heads at temperatures in the range 45° C. to 75° C., or even higher, and said well heads are often horizontally several kilometers away from the surface support that is to receive and process the crude oil, while the sea water is at a temperature of about 3° C. to 5° C. Furthermore, in a depth of water reaching or exceeding 2000 m to 3000 m, it is desirable to keep the crude oil at a temperature higher than 30° C.-35° C. until it reaches the surface in order to avoid forming plugs of paraffin or of gas hydrates, which would block production. This therefore requires continuous high performance thermal insulation of the bottom-to-surface connection pipe conveying the crude oil.
Numerous types of insulated pipe have therefore been developed, and in particular so-called pipe-in-pipe (PiP) type pipes in which an inner pipe conveys the fluid and an outer pipe arranged coaxially around the inner pipe, also known as the “outer jacket”, is in contact with the surrounding medium, i.e. water. The annular space between the two pipes may be filled with an, insulating material or it may be evacuated.
Those systems have been developed to achieve a high degree of thermal performance and specific versions have been developed so as to be particularly adapted to great depths, i.e. so as to be capable of withstanding the pressure at the sea bottom. Since the pressure of water is substantially 0.1 megapascals (MPa), i.e. about 1 bar, for every 10 m of depth, the pressure that the pipe needs to be capable of withstanding is about 10 MPa, i.e. about, 100 bars, for every 1000 m of depth, and about 30 MPa or about 300 bars for 3000 m.
Such high performance pipes are used to constitute the main running lengths both of pipes resting on the sea bottom and of bottom-to-surface connection pipes, but in general they are not suitable for use in constituting singular junction elements known as “spool pieces” or indeed junction pipes, since such pipe elements generally present shapes that are complicated, including a plurality of bends that need to be fabricated after the undersea pipes and the bottom-to-surface connection installation have been laid.
Outer pipe insulating means are known that withstand high hydrostatic pressures and that are therefore suitable for being used when immersed at great depths, being constituted by:                quasi incompressible solid polymer material coverings based on polyurethane, polyethylene, polypropylene, etc., that, where appropriate, are in the form of a solid tubular sleeve. However, such materials present thermal conductivity and thermal insulation properties that are relatively poor, and insufficient to avoid the above-mentioned drawbacks of plugs forming in the event of production being stopped in an undersea pipe conveying hydrocarbons; or        coverings of synthetic materials made up of hollow beads containing a gas and capable of withstanding the external pressure, the beads being embedded in binders such as concrete, epoxy resin, and in particular coverings known as syntactic foams, etc., having thermal insulation properties that are better, but that are considerably more expensive and more difficult to fabricate and to install. Recourse is made to half-shells that are assembled around the assembly weld that needs to be protected after the welding has been performed. However, it is then necessary to fill the gape between the shells in such a manner as to avoid forming any localized thermal bridges. In another embodiment, the pipe or pipe portions have insulation molded directly thereon so as to obtain insulation without discontinuity at the periphery, however, that method presents the drawback of being the subject of cracking due to the large temperature gradient between the pipe, generally at a temperature in the range 50° C. to 85° C., and the surrounding sea water in contact with the outer jacket, which is at a temperature of 3° C. to 5° C.        
Insulating materials are also known that have better thermal insulating properties, i.e. lower thermal conductivity associated with phase-change properties. Insulating phase-change materials (PCM) are implemented in particular in WO 00/40886 and WO 2004/003424, however, those insulating PCMs that are capable of adopting a liquid state need to be confined in an absorbent material, as described in WO 00/10886 or they need to be confined in pockets, as described in WO 2004/003424.
Those thermally insulating coverings are themselves covered in a semirigid continuous tubular outer jacket. However, in the prior art, the embodiments described are limited to fabricating straight pipes and they are not adaptable to fabricating pipes with bends. Those embodiments are not adaptable to thermally insulating junction pipes with bends because of the structure of the outer jacket, which as described cannot be deformed to extend coaxially about the inner pipe and does not enable a substantially constant thickness of insulating material to be obtained, in particular in bend zones.
Other insulating materials in gel form have also been described, in particular in patents FR 2 800 915, FR 2 820 426, and FR 2 820 752. More particularly, those insulating gels are constituted by a complex comprising a first compound that presents thermal insulation properties and that is mixed with a second compound consisting in a gelling or structuring-effect compound, in particular one that operates by curing, such as a polyurethane compound, said first compound being in the form of particles or microcapsules that are dispersed within a matrix of said gelled or cured second compound, said matrix thus confining said insulating first compound that might possibly be in the form of a liquid, thereby greatly reducing convection phenomena.
Said first compound may itself be a phase-change compound such as paraffin, other compounds in the alkane family, such as waxes, bitumens, tars, fatty alcohols, glycols, and still more particularly any compound having a melting temperature lying between the temperature t2 of the hot effluent flowing in the inner pipe and the temperature t3 of the medium surrounding the pipe in operation, i.e. in general a melting temperature lying in the range 20° C. to 80° C.
However, the first compound may be an insulating material that does not change phase, such as kerosene.
In prior embodiments, such insulating gels are confined between a steel inner pipe and a flexible or semirigid outer protective jacket, but only for pipe portions that are straight.
For that purpose, previously-prepared tubular jackets are placed on one another by being threaded coaxially, and the gel is injected into the annular space after the ends of said tubular jackets have been closed.
That method is not applicable to pipe portions having bends, since the flexible outer jacket cannot be bent to the required radius of curvature without being folded or kinked, thereby causing the cross-section of the outer jacket to lose its circular and concentric shape about the inner pipe. That gives rise to reductions in the thickness of the inside annular space between the inner pipe and the outer jacket and thus to localized reductions in thermal insulation through said annular space.